Electroluminescent phosphors



Dec. 26, 1961 L. BURNS 3,014,873

ELECTROLUMINESCENT PHOSPHORS Original Filed Aug. 20, 1952 WEEK/1450MB? LEVELS WME/VLE BAND 1 INVENTOR Unite rates ice 3,014,873 ELECTROLUMINESCENT PHOdPHORS Laurence Burns, Swampscott, Mass, assignor, by mesne assignments, to Sylvania Electric Products Inc, Wilmington, DeL, a corporation of Delaware Original application Aug. 20, 1952, Ser. No. 305,400, now Patent No. 2,755,406, dated July 17, 1956. Divided and this application June 29, 1956, Ser. No. 594,742

3 Claims. (Cl. 252301.4)

This invention relates to electroluminescent lamps, that is to lamps using phosphors which lumlnesce when placed in an electric field.

The phosphor in such lamps is suspended or embedded in a solid or liquid dielectric medium, placed between two electrodes, one of which is transparent. I find that the dielectric medium can be substantially eliminated if the powdered phosphor particles are pressed together directly between two electrodes, between which a voltage on be applied.

The elimination of the embedding dielectric medium reduces the possibility of electrical breakdown in the dielectric, and permits the use of higher applied field strengths. Fields between 16 and 10" volts/cm. can then be applied, which tend to increase the efiiciency. In particular, the high field enables the excitation of electrons directly to the excited activator levels in the phosphor, from which they emit light on return. The electron does not have to be raised to the conduction band and then ccelerated to an energy sufiicient to excite an atom by collision or the like. The losses consequent to the electrons travel through the phosphor are thereby avoided, with consequent improved efiiciency.

To achieve such a result, the activator shouldbe a substance which can be raised to a level sufiicient for light emission on return, but below the bottom level-of the I is filtered and washed on the filter with two portions of proportion of about 0.005 mole of the manganese carbonate to one mole of the sulfide. The addition of a small amount of zinc oxide, say about 0.1 mole per mole of sulfide, will generally increase the brightness somewhat.

The mixture is then fired by being moved at a rate of about an inch per minute through a six-foot silica tube, three inches in diameter with a quater-inch wall, and heated over its middle section, for a length of about four feet, to a temperature of about 1720 C. The mixture passes through the tube in trays six inches long, two inches wide and one and one half inches deep. One end of the silica tube is left open, and the trays are fed into that end; the other end is closed by an'air lock with a sliding gate at each end to avoid disturbing the flow of inert gas through the tube when trays are removed. The gas enters the tube through a pipe at the closed end and flows in a direction opposite to that of the movement of the trays.

an aqueous /fa% acetic acid solution, and then washed twice with distilled water.

Mixing a small amount of zinc chloride with the zinc sulfide, say in the proportions of 0.5% by weight before firing will sometimes improve the light emission. Some ot the chloride willbe lost on firing with only about conduction band, and should be a substance in which dilower fields, an additional activator such as copper has to be used, to provide electrons in the conduction band which can be accelerated to excite the manganese activator. This acceleration, however, with consequent travel of the electrons through the crystal, introduces losses to the main lattice material of the crystal.

By using fields high enough to cause excitation of the activator to an intermediate level, but not high enough to excite many electrons to the conduction band, such losses can be greatly reduced and the efficiency of the crystal thereby improved.

Gther features, objects and advantages of the invention will be apparent from the following specification, taken in connection with the accompanying drawings which:

FIG. 1 is a cross-sectional view of one embodiment of the invention; and FIG. 2 is a schematic diagram of energy levels in one embodiment of an electroluminescent phosphor of the invention. 7

In FIG. 1, the glass plate lhas the light-transmitting, electrically-conductive coating 2, over which is the cornpressed layer 3 of powderedelectroluminescent phosphor,

withthe metal backing layer 4 thereover. A mica piece 5 is placed over the metal layer 4, and extends beyond the edgesthereof to protect the phosphor layer 3. A layer 6 of plastic ,or ceramic material encloses the backof the device and is sealed to the glass plate 1 around the edges of the latter.

The phosphor powder of the layer 3 is made by mixing 0.01% remaining in the fired mixture. Another chloride than zinc, for example, manganese chloride or ammonium chloride can be used, as long as the proportions are kept within the proper limits.

The manganese can be added in other forms, for example, as the oxide, a compound reducible to the oxide on heating, the chloride, and the like. The manganese content is preferabl-y ke'pt within the range of 0.001 to 0.03 gram-atoms of manganese per mole of zinc sulfide. Part or all of the zinc sulfide can be replaced by the selenide, if desired. A combination of the two can help to increase the local field strength at certain points in the crystal. The phosphor particlescan be about 10 microns in dimension, and the phosphor layer can have a thickness of many particles.

the device."

The conductive coating 2 on glass plate 1 can be one of those known in the art, and may be applied for example, by exposing the heated glass to vapors of silicon, tin or titanium chlorides, and afterwards placing it in a slightly reducing atmosphere. In some cases, stannic chloride is mixed with: absolute alcohol and glacial acetic acid and the glass plate to be coated dippedinto it, if the application by vapors is not convenient.

Whether applied by vapor, dipping or otherwise, the resulting coatingappears to contain stannic '(or silicic or titanic) oxide, probably to some extent at least reduced to a form lower than the dioxide, although the exact composition is not fully known. I

The metal layer 4 may be applied as a foil, a conductive metal paint, or may be' vacuum-evaporated onto the coating 3, taking care however, not to let the metal get farenough into the phosphor coating 3 to short circuit It is important that the conductive layers 2 and 4 be in good. electrical contact with the phosphor layer 3. Forthis reason, the rr'ietalbacking layer 4 should be coating 2 z nc sulfide with manganese carbonate as powders, in the Plastic material 6 may be molded around the back c) of the device, being sealed to the glass plate 1, preferably at the edges of the latter as shown. The plastic is preferably sealed under pressure, and may be a thermosetting material suchas the usual phenolic or melamine resins,

some of the plastic extending to the front of the glass plate .1 around the edges thereof as shown, to clamp the various pieces 1, 2, 3 and ,4 together under sufficient pressure to secure good contact.

The-phosphor layermay be a 'few-t-housandths of an inch, say 0.004 inch thick, which with about 10,000 volts across it, would correspond to an average field of about .10 volts/cm.

In providing the phosphor layer 3, the phosphor powder .can be. spread out overglass platel and conductive coating.-2,' with a mask placed over the area which the layer .is not to, occupy, and pressure applied to compress the powder, the pressures applied being as high as possible without breaking the .glass. The phosphor layer itself, when not limited by the type of glass used, can be compressed at least as high as 9,000 pounds per square inch.

The phosphor layer 3 should extend beyond the edges of the conductive coating 2 and the metal backing 4, in order to make the leakage path between the two conductors 3, 4, as long. as necessary to prevent flashover. The phosphor layer can extend an-eighth inch or more beyond the conductors .3, 4, for convenience. In some cases a thin insulating layer 5, for example, of mica, is placed over the metal layer 4, to protect the phosphor layer 3 for a distance of say one-eighth inch or so beyond the metal layer, to prevent harm to the phosphor layer in that region when theplastic 6 is'applied. The

preferably rounded ofi-at its edges.

,The field in the crystal will differ from the average .field between electrodes, because of irregularities in the crystal shape and in the composition of the crystal from point to point. The field in the crystal can be enhanced also by mixing with the phosphor particles in the phosphor layer 3, a small quantity of metal particles, which can be'of somewhat the same size as the phosphor particles. .Five to ten percent by volume of metal particles will be sufficient, because a-larger percentage will be -more likely. to provideshort-circuiting effects between the electrodes or shielding effects for the crystals. ,Another way in which such ingredients can beprovided is by taking acom-pressedlayer of the powdered phosphor and evaporating onto it a very thin metal film, say a micron thick-or even less, then ball-milling the phosphor again to breakup the layer and provide crystals, part of whose surface is free from metal and part of which has a metal coating. Such -an arrangement is especially useful because advantage can be taken of the field at the metalcrystal contact.

However, the proportion of partly-metalcoated crystals should bekept small enough with respect to the non-coated crystals, or the proportion of coated surface on each crystal should be kept small enough, to

prevent short-circuiting of the phosphor layer, by either direct short-circuit or by arcing.

In-some cases, the field required may be low enough 'so thatelectroluminescence can be obtained from phosconduction band, an improvement in efiiciency may be obtained'at very ,highfields such as those between and 10 volts/cm.

The phosphor can be used in the powdered layer form described above. To get the full benefit from the increased field, the-amount of copper used should be in- 0.01 are desirable. Correspondingly increased lead and chloride contents are also helpful.

In FIGURE 2, a schematic representation of the crystal energy diagram for the phosphor is shown, the abscissae representing distance through the crystal and the ordinates representing energy. The lower band V is the valence band. The type band C is the conduction band, whose levels are normally unfilled, but to which electrons from the valence band can beexcited' by-sufiiciently high fields. In between these bands which extend across the entire crystal are the intermediate levels, I and 1 which exist only at-certain parts in the crystal, generally parts immediately around an activator impurity atom.

phosphor layer 3extends beyond the mica layer and is When an electron is excited from the valence band V or say the intermediate level 1 to the conduction band C, it can travel some distance along the crystal in that band, losing energy by scattering effects, by collisions with other atoms, and the like. Eventually it may drop back to a lower level, emitting light in the process, but the efiiciency of the emission is reduced by the losses.

If, however, the electron from I is excited, not all the way to the valence band, but merely to the higher intermediate level .1 it will not travel along the crystal, and will thus avoid losses of energy in collision with other atoms and the like. The process will be one of higher efiiciency; the electron will simply be excited from one level in the activator region to another level in the same regions, and the efiiciency will accordingly be more nearly comparable with that achieved by excitation of an activator from one level to another by radiation. The field excitation process,willstillhave some dielectric losses, unless DC. .is used, but the conduction losses will be smaller.

,Although manganese activated zinc-sulphide is given above as a specific example of a phosphor excitable to an ,activatorlevel without being excited to the conduc- -tion band otherphosphors may be used, for'example,

tin-activated calcium phosphate or manganese-activated zinc or cadmium silicate. Thezinc in the. zinc sulphide described can-.be replaced in whole or in part by cad- .mium, and the sulphide can be replaced in whole or in part byselenium.

'The amount of manganese activation in the sulphides cannbe much higher than the limit of the preferable Although some improvement in eificiency can'be obtained 'withincreased-field strength in copper-activated phosphors, 'or in other phosphors in which electrons travelin the conduction band, if the content of copper or similar activator is increased, the greatest increase in efiiciency will be obtained when the phosphor is free from copper and any other materials which facilitate the entry of electrons into'the conduction band.

, Thisapplication is a division of applicants copending application Serial No. 305,400, filed August 20, 1952,

for Electroluminescent Lamps. That application was issued on July 17, 1956, as United States Patent2,755,406.

What I claim is:

1. The method of making an electroluminescent phosphor which comprises applying a thin metal film onto a powered layer of inorganic phosphor, and then breaking up the layer into particles.

2. The method ofmaking an electroluminescent phosphor which comprises evaporating a thin metal film onto a powdered layer of inorganic phosphor, and breaking up the layer into particles.

creased above the 0.001 gram-atoms value used per mole qof-zincsulfideat lower ,fields. Cpppencontents-of about ,3. The method of making an electroluminescent phosphor which comprises applying a thin metal film onto a powdered layer of inorganic phosphor, and then ballmilling the. layer to break it up into particles.

(References on following pag References Cited in the file of this patent UNITED STATES PATENTS Fischer Aug. 4, 1936 Nelson Dec. 1, 1942 Mager Sept. 4, 1951 Mager Jan. 6, 1953 Kollcr May 31, 1955 6 2,755,400 Stiles July 17, 1956 2,755,406 Burns July 17, 1956 2,857,541 Etzel Oct. 21, 1958 OTHER REFERENCES Burns: Electrcluminescence of Insulated Particles, Jour. of Electrochem. 800., December 1953, vol. 100, No. 12, pp. 572 to 579. 

1. THE METHOD OF MAKING AN ELECTROLUMINESCENT PHOSPHOR WHICH COMPRISES APPLYING A THIN METAL ONTO A POWERED LAYER OF INORGANIC PHOSPHOR, AND THEN BREAKING UP THE LAYER INTO PARTICLES. 